Radio frequency pulser



June 12, 1956 w. A. WELSH ET AL 2,

RADIO FREQUENCY PULSER Filed Nov. 28, 1952 INVENTORS 71 x77? Wlak Ezbfiard BY I ATTORNEY 2,750,530 more FREQUENCY PULSER Warren A. Welsh and Richard C. Tardifi, Winston- Saleni, N. (3., assignors to Western Electric Company, lgicolii'porated, New York, N. Y., a corporation of New Application November 28, 1952, Serial No. 322,938

7 Claims. (Cl. 315-) This invention relates to radio frequency pulsers, and more particularly to microwave frequency generators which have an output of continuous wave energy or modulated pulses.

One object of the present invention is to provide a microwave signal generator, the output pulses of which have a repetition rate equal to the frequency of the input modulating pulse.

Another object is to provide a microwave signal generator in which the spacing between pulses is not particularly critical and does not react upon signal strength.

Another object is to provide a signal generator primarily intended for low duty cycle pulse operation and which is easily calibrated in terms of peak output.

Another object is to provide a signal generator in which the modulating pulse requirements are far less stringent than is usual, especially with regard to amplitude.

With these and other objects in view, the invention comprises an oscillator of the reflex Klystron type that is set into a predetermined mode of continuous wave operation when a negative voltage is applied to a voltage dividing network to which a repeller plate of the Klystron oscillator is connected, a source of modulating pulses which are applied to the primary of a transformer, the secondary of which is in series with a switch and a diode, the latter series combination shunting a selected resistor of the voltage dividing network, the modulating pulse being applied when the switch is closed and the selected resistor is effectively short circuited to ground through the diode, the transformer secondary, and the switch; and the selected resistor being reintroduced into the circuit when a portion of the modulating pulse that has sufficient negative amplitude cuts off the diode, thereby placing the Klystron oscillator again in the predetermined mode of continuous wave operation as long as the modulating pulse is sufliciently negative.

Other objects and advantages will be apparent from the following detailed description when considered in conjunction with the accompanying drawing wherein the single figure shows schematically a microwave signal generator embodying the features of the present invention.

Referring now to the drawing, the numeral 10' designates a microwave signal generator of the reflex Klystron type having a grounded cathode 11 for supplying a stream of electrons which are accelerated toward the repeller plate 12 by a positive potential of 300 volts with respect to ground potential on the resonant cavity 14-. The repeller plate 12 has a negative potential applied to it by a negative voltage of 300 volts with respect to ground potential applied across a voltage divider network consisting of a potentiometer 22 and fixed resistors 24, 26, and 27. This negative potential on the repeller plate 12 forces the electrons to retrace their path back through the grids 16 and 17 to provide in the resonant cavity 14 oscillatory energy of a frequency and output power depending upon the repeller plate-to-cathode potential, the

2,750,530 Patented June 12, 1956 physical size of the resonant cavity, dimensions of the tube and so forth. This output energy is taken from the resonant cavity 14 by a coupling loop 18 connected in the resonant cavity 14 and an associated output coaxial line consisting of an inner conductor 19 and an outer conductor 20. I

A modulating pulse is applied to the junction of the resistors 26 and 27 of the voltage divider network and ground in the following manner: a line 28 is connected from the grounded end of the resistor 27' to one end of a switch 29. The other end of the switch 29 is connected by a line 30 to one end of the secondary of a transformer 31, the other end of which is connected by a line 32 to the anode 34 of a diode 36. The cathode 37 of the diode 36 is connected by a line 38 to the other end of the resistor 27, to which one end of the resistor 26 is also connected.

To the primary of the transformer 31 is connected a source of modulating pulses 39. The modulating pulses generated by this source have alternately positive and negative amplitudes and frequencies in the order of the radio frequencies. These modulating pulses are coupled through the transformer 31 to a load resistor 40 which shunts the secondary of the transformer 31.

A capacitor 41 of a low value may be connected between the cathode 38 of the diode 36 and the repeller plate 12 of the Klystron 10 to correct any wave form distortion of the voltage applied to the repeller plate 12 of the Klystron 10.

In the operation of the above circuit, a positive 300 volts is applied to the resonant cavity 14 and a negative 300 volts is applied to one end of the potenti'ometer 22 of the voltage divider network. With the switch 29 closed, the resistor 27 is effectively shortcircuited through the diode 36 (normally conducting), the secondary of the transformer 31, and the switch 29 to ground. The value of the resistors 24 and 26 together with the setting of the potentiometer 22 are selected so that the negative voltage applied to the repeller plate 12 prevents oscillation of the Klystron 10. With the switch 29 open, the resistor 27 is no longer, in effect, shortcircuited and the value of resistor 27 together with values of resistors 24 and 26 and the setting of potentiometer is selected to give the desired continuous wave (CW). operation of the Klystron 10.

With the switch 29 closed (non-oscillatory condition) and a pulse of suflicient amplitude and of negative polarity from the pulse source 39 applied to the primary of the transformer 31 and hence across the resistor 40, the anode 34- of the diode 36 is driven negative with respect to the cathode 37 thereby rendering diode 36 non-conductive. Thisis equivalent to opening switch 2? and? the Klystron oscillator 10 will provide the same output as if the switch 29 were manually opened.

The output of the oscillator 10 is independent of the modulating pulse amplitude or flatness of top since the diode 36 will be cut-01f when the anode 34 is negative with respect to the cathode 37. With this condition theeffective pulse that appears at the cathode 37 of the diode 36 will be a negative pulse equal in amplitude tothe voltage across the resistor 27 with the switch 29 open: and of the same time duration as the negative portion of the modulating pulse when the modulating pulse has a negative amplitude equal to or greater than the negative voltage across the resistor 27 with the switch 29 open. The pulse applied to the repeller plate 12 of the Klystron 10, therefore, has a flat top and hence there is no frequency shift in the output of the Klystron it! during the pulsing. However, primarily due to distributed capacitance of the circuit, the applied pulse is not perfectly flat and some shift may occur. The judicious addition of a capacitor of low value such as shown at 41 will correct wave form distortion caused by this distributed capacitance. This is especialiy desirable for fast-rise-time, short-duration pulses.

Since a portion of the negative part of the input modulating pulse sets the Klystron back into its predetermined mode of continuous wave operation, the microwave signal pulses at the output of the Klystron have a' repetition rate equal to the wave frequency of the input pulse.

It is obvious from the description above that the voltage of the repeller plate 12 will be exactly the same during the period of the pulse in which the anode 34 of the diode 36 is negative with respect to the cathode 3'7 with the switch 29 closed as when the switch 29 is open. Therefore, since the biasing of the Klystron It) is exactly the same under continuous wave (CW) and pulse modulated conditions, the same frequency and power will be obtained under these two conditions. Hence, the oscillator can be adjusted for frequency and power on CW then switched to modulation with complete assurance of proper operation,

The adjustment of the frequency and power of the Klystron 10 on CW is important because under pulse conditions, the Klystron supplies only very small amounts of power due to the low duty cycle of, for example, 1 in 500. Meters to measure this extremely small amount of power are not readily available. However, when the peak power measurements are made while operating the oscillator on continuous wave (CW), since the average power of a CW wave is also its peak power, the level would be some 500 times the average under pulse modulation, and this higher level, which is in the order of 1 milliwatt, is capable of measurement on many ordinary power meters.

The changing of the circuit from the CW to the pulse modulated condition or vice-versa is readily accomplished by switch 29 which serves two functions-primarily it allowsthe pulser to be operated on CW for calibration and adjustment, and secondly it permits the circuit to be used as a source of a CW signal.

Since the circuit does not require charging or discharging times in excess of the minimum spacing between pulses and since the peak currents required in the oscillator do not exceed those required for CW operation, the circuit is capable of modulation with closely spaced pulses of the order of, for example, .5 microsecond in duration with no interaction upon signal strength between pulse groups.

It is to be understood that the above described arrangement is merely illustrative of the principles of the invention and that numerous other modifications may readily be devised by those skilled in the art which fall within the spirit and scope thereof.

What is claimed is:

1. In a radio frequency pulser; a cavity-type oscillator having a repeller plate, the voltage of which is one factor in determining the mode of operation of the oscillator; means to supply a negative voltage of predetermined magnitude to the repeller plate to put the oscillator in a predetermined mode of continuous wave operation; means to apply a modulating pulse of a predetermined frequency to the repeller plate; said modulating pulse means including a diode; a portion of said modulating pulse cutting off the current flow through said diode to effectively cut off the source of the modulating pulse with respect to said repeller plate to automatically place the oscillator in the said predetermined mode of continuous wave operation.

2. A microwave signal generator comprising a cavitytype oscillator having a repeller plate, the voltage of which determines the mode of continuous wave operation at which the oscillator operates, a voltage divider network consisting of a plurality of resistors connected to the repeller plate, means for applying a voltage of predetermined magnitude to the voltage divider network to place the oscillator in a predetermined mode of continuous wave operation, means to apply a modulating pulse to at least one resistor of the voltage divider network, said modulating pulse applying means including a source of modulating pulses having positive and negative amplitudes, means to place low impedance elements in shunt with a selected resistor of the voltage divider network to place the oscillator in a non-oscillatory condition, the selected resistor being reintroduced into the circuit when the modulating pulse source emits a negative pulse thereby placing the oscillator in said predetermined mode of continuous wave operation and permitting the oscillator to produce a series of microwave signal pulses having a repetition rate equal to the wave frequency of the modulating pulse.

3. In a microwave signal source; a microwave oscillator of the type having a cathode, a resonant cavity, two grids, and a repeller plate, the voltage of which is one factor in determining the mode of operation of the oscillator; said repeller plate being connected to the junction of two resistors of a voltage divider network consisting of a plurality of resistors; a source of voltage negative with respect to the cathode to energize the voltage divider network and hence apply a negative voltage of predetermined magnitude to the repeller plate of the oscillator to operate it in a predetermined manner; means to apply modulating pulses across a selected resistor of the voltage divider network; said modulating pulse applying means including a transformer, a modulating pulse source for applying to the primary of the transformer a pulse having positive and negative amplitudes, a resistor across the secondary of the transformer, a switch connecting one end of the secondary of the transformer to one end of the selected resistor of the voltage divider network, and a diode having its anode connected to the other end of the transformer secondary and its cathode connected to the other end of the selected resistor of the voltage divider network.

4. A microwave signal generator comprising an oscillator of the reflex Klystron type having a cathode to supply a stream of electrons, a repeller plate, a resonant cavity, and two grids; said grids used for receiving the electrons when they are repelled from the repeller plate; a coupling loop attached in the resonant cavity to conduct the output therefrom; a source of positive potential applied to the resonant cavity; a voltage divider network comprising a plurality of resistors connected to the repeller plate; a source of negative potential connected to and energizing the voltage divider network; a modulating circuit connected across at least one of the resistors of the voltage divider network, said modulating circuit including a modulating pulse source, a transformer, a resistor across the secondary of the transformer, a switch, and a diode; one end of the transformer connected through the switch to one end of the said resistor of the voltage divider network, said end of said resistor being grounded, and the other end of the transformer connected to the anode of the diode, the cathode of which is connected to the other end of the resistor of the voltage divider network; means for adjusting said voltage divider network to put the oscillator in continuous wave operation with the switch open, said modulating pulse permitting the diode to conduct when the switch is closed and the modulating pulse is positive and cutting off the diode when the modulating pulse is negative thereby biasing the oscillator the same for pulsed operation as for continuous wave operation.

5. In a microwave signal generator, a modulating pulse emitting means, a diode, a transformer having its primary connected to the modulating pulse emitting means and one end of its secondary connected to the anode of the diode, a switch connected to the other end of the secondary of the transformer, a shunting resistance connected across the secondary of the transformer, means for connecting the switch when closed to ground, a resistor connected across the said switch grounding means and the cathode of the diode, a plurality of series connected resistors having one end connected to the cathode of the diode, a voltage source having its negative terminal connected to the other end of said series connected resistors, said diode in conjunction with said transformer secondary and said switch shunting a low impedance across the resistor connected across the switch grounding means and the cathode of the diode, an oscillator having a resonant cavity and a repeller plate, a conductor connected between two of the said series resistors and the said repeller plate of the oscillator, means for connecting the resonant cavity to the positive terminal of a voltage source, an output loop means coupled to the resonant cavity, and means for energizing the oscillator to transmit continuous wave energy or modulated pulses from the resonant cavity to the output loop means as controlled by the said repeller plate,

6. In a microwave signal generator, an oscillator of the type having a cathode and an electrode the voltage of which determines the mode of operation of the oscillator, a voltage divider network having a plurality of resistors, a source of voltage which is negative with respect to the cathode applied across said network, said electrode connected to a junction of two of said plurality of resistors, a source of modulating pulses adapted to be connected across a third of said plurality of resistors, and means to shunt to ground with circuit elements of low impedance said third resistor, a portion of said means being adapted 6 to cut off said source of modulating pulses from across said third resistor.

7. A microwave signal generator comprising an oscillater of the reflex Klystron type having a cathode at a reference potential and a repeller electrode, the voltage of said repeller electrode determining the mode of operation of said oscillator, a plurality of serially-connected resistors, means to apply a negative voltage across said resistors, said repeller electrode being connected to a junction of two of said resistors, the series combination of a switch, the secondary of a transformer, and a diode, said series combination connected across a third of said plurality of resistors so that said third resistor has a low impedance shunt to ground when said switch is closed and said diode is conducting, and a source of modulating pulses connected to the primary of said transformer, said diode being adapted to be rendered nonconductive by a portion of said modulating pulses.

References Cited in the file of this patent UNITED STATES PATENTS 2,482,769 Harrison Sept. 27, 1949 2,511,789 Rosencrans June 13, 1950 2,595,662 Houghton May 6, 1952 2,601,096 Creamer et a1. June 17, 1952 FOREIGN PATENTS 121,739 Austria July 11, 1946 

